Step 4 · Apply Flux

With the pipe and fitting cleaned, use a dedicated flux brush to apply a thin, even layer of lead-free soldering flux to the cleaned outer surface of the pipe. Also apply a thin, even coat to the cleaned inner surface of the fitting. The flux serves two purposes: it chemically cleans the copper as it's heated, preventing re-oxidation, and it helps draw the molten solder into the joint via capillary action. Avoid using too much flux, as excess can run inside the pipe and cause corrosion over time. Never use your fingers to apply flux.